1. Field of the Invention
The present invention relates to a silicon carbide sintered body and a process for making the same and, more particularly, to a silicon carbide sintered body having a high density useful for structural parts and components of semiconductor manufacturing equipment, electronic information equipment, vacuum devices, and the like.
2. Description of the Related Art
Silicon carbide is a highly covalent material and has found various applications taking advantage of its properties of, for example, strength at a high-temperature, heat resistance, wear resistance and chemical resistance. Recently, there have been demands in the fields of materials for electrical information equipment and semiconductor manufacturing toward a high heat-resistance, and a highly dense and highly pure silicon carbide sintered body without heat distortion and due to heat deterioration when washed with chemicals such as hydrofluoric acid, as in conventional parts and components made of quartz with increase in wafer processing temperature, wafer diameter, and processing unit.
As mentioned above, the silicon carbide is highly covalent and is thus difficult to sinter. Dense silicon carbide sintered bodies are known to be produced by means of hot pressing, reaction sintering, and atmospheric sintering.
The hot pressing is a method in which silicon carbide is sintered under a high pressure and has been studied with a variety of metal sintering additives after this technique was reported (J. Am. Ceram. Soc., 39(11), 386-389 (1956)) with aluminum added as a metal sintering additive. A highly conductive and electrically insulating sintered body was developed in 1980 that was sintered by the hot pressing by adding BeO("Silicon Carbide Ceramics", pages 327-343 published by Uchida Rokakuho (1988)).
The reaction sintering involves in the following steps of: (1) mixing raw materials (i.e., mixing silicon carbide powder and carbon powder), (2) molding fabricating, (3) reaction sintering, and (4) post-processing, if necessary. This technique involves silicification of the carbon particles which have already been molded in the reaction sintering step (3) above, which provides an advantage of allowing sintered bodies of less variation in dimension without any sintering additives. Accordingly, this technique provides an easier operation to produce a sintered body of a higher purity and has been used for production of parts and components for semiconductors. However, the sintered bodies obtained by means of this technique contains non-reacted metal silicon so that their applications are restricted when parts and jigs are used in the fields where the heat resistance, the chemical resistance and a high strength are required.
The atmospheric sintering process is a technique characterized by using sintering additives for sintering the silicon carbide and was proposed by S. Prochazka in "Ceramics for High Performance Applications" in 1974, on page 239. This technique allows production of a high dense structural member having a high strength at a high temperature. As a result, studies of the silicon carbide have been developed. The sintering additive used is a combination of a metal-based sintering additive comprising a metal such as boron, aluminum and beryllium or a compound thereof and a carbon-based sintering additive such as carbon black and graphite. The metal-based sintering additive has essential effects of, for example, reduction of surface energy at a grain boundary due to local deposit of boron onto the boundary, enhancement of diffusion of carbon-boron substances on the grain boundary, and suppression of surface diffusion thereof, for the boron which is used as the optimum sintering additive. The carbon-based sintering additive is considered to have an effect of removing an oxide layer on the surface of silicon carbide particles. Details, however, have still remained unknown.
Metal contaminants are eluded when the metal-based sintering additive used is exposed to a high temperature or is subjected to washing with chemicals. The resultant sintered bodies are thus not suitable for the application to the areas of the semiconductor manufacturing equipment.
In order to overcome the above mentioned problems, Japanese Patent Application Laid-Open (JP-A) No. 60-108370 proposes a process for making a dense sintered body by hot press process without adding a sintering additive, using a special ultra-fine powder of silicon carbide obtained through heat decomposition of a silane compound. However, there is no clear description on properties of the resultant sintered bodies. In this connection, "Silicon Carbide Ceramics", published by Uchida Rokakuho, in 1988, on page 89, describes that it is essential to add boron as a metal-based sintering additive even by using the powder obtained according to this technique.
As an improved hot pressing, Japanese Patent Application Laid-Open (JP-A) No. 2-199064 proposes a process for making a dense sintered body without any additives by the hot press method, using ultra-fine silicon carbide powder synthesized by means of CVD plasma technique. However, impurities such as iron are contained in an amount of several ppm or more even in the process described in this application. This is not a satisfactory level of impurities. The ultra-fine silicon carbide powder used for this system as the sintering additive has an average particle diameter of 30 nm. Such ultra-fine powder is relatively expensive and should be treated with significant care for preventing oxidation on the surface thereof. In light of the above, the process disclosed in the above application is far from a solution to the problems to date.
It is difficult for the conventional processes to obtain a high dense silicon carbide sintered body, containing less or no impurities, which is suitable for being used for parts and components of semiconductor manufacturing equipment and electronic information equipment. Furthermore, there is no such a sintered body available on the market.